Liquid crystals have excellent characteristics from various points of view, namely,
1) they can be operated at a low voltage;
2) they consume only small amount of power;
3) panel type displays can be used; and
4) they are passive type matrixes.
Accordingly, electrooptical apparatuses using nematic liquid crystals such as a DSM cell, TN cell, G-H cell, or STN cell have been developed and practically used. However, all of the electrooptical apparatuses using such nematic liquid crystals have a defect that response time is as slow as several milli sec to several tens milli sec, leading to a restriction in their applications.
In order to solve these problems, active matrix drive systems using STN cells or thin-film transisters were developed. However, STN type displays have problems that a high accuracy is required in controls of cell gap and tilt angle and that response time is rather slow, while they are excellent in such qualities as display contrast and viewing angle.
With such a technical background, the development of a ferroelectric liquid crystal had been attempted which has a spontaneous polarization (Ps), has a strong torque based on Ps.times.E (E is an applied voltage), and has an extremely short optical response time of a few .mu.sec to several tens of .mu.sec to make the preparation of a hypervelocity device possible.
Meyer et al. synthesized DOBAMBC (p-decyloxy-benzilidene-p-ammino-2-methylbutyl cinnamate) in 1975 for the first time in the world and which was confirmed to be a ferroelectric liquid crystal (Le Journal de Physique, Vol. 36, 1975, L-69).
Further, since Clark and Lagawall reported in 1980 on such characteristics on display devices as high velocity response of submicroseconds and memory characteristics of DOBAMBC, ferroelectric liquid crystals have absorbed considerable public attention (N. A. Clark et al., Appl. Phys. Lett. 36, 899 (1980)).
However, from a practical standpoint, there were many technical problems in that system. In particular, no material exhibited ferroelectric liquid crystallinity at an ambient temperature, and an effective and practical method was not established to control the molecular alignment of the liquid crystal molecules. Controlling the molecular alignment of liquid crystal molecules is essential for applications in liquid crystal devices.
After the publication of the report, various attempts have been made from both aspects of liquid crystal materials and device. Display devices utilizing the switching between twisted bistable states were prepared for trial, and high speed electrooptical apparatuses using the device are proposed in U.S. Pat. No. 4,367,924 and others. However, high contrast and proper potential of threshold value have not been obtained.
From such a point of view, other switching systems were explored to propose a transitional diffusion system. subsequently, a three states switching system of liquid crystal having tristable states was reported in 1988 (A.D. L. Chandani, T. Hagiwara, Y. Suzuki et al., Japanese J. of Appl. Phys., 27, (5), L729-L732 (1988)).
The optically tristable states herein referred to mean that, when voltage in the form of a triangular wave as shown in FIG. 1A is applied to liquid crystal electrooptical devices where antiferroelectric liquid crystals are laid between the first electrode substrate plate and the second electrode substrate plate which is apart at a given space from the first one, the antiferroelectric liquid crystals show the first stable molecular orientation and resulting the first optically stable state as shown in FIG. 3 (a), and FIG. 1(D) at reference point 2, respectively, when electric voltage is zero. The antiferroelectric liquid crystals show the second stable molecular orientation and resulting the second optically stable state as shown in FIG. 3 (b), and FIG. 1(D) at reference point 1, respectively, in one of the direction of electric field; and show the third stable molecular orientation and resulting the third optically stable state as shown in FIG. 3 (c), and FIG. 1(D) at reference point 3, respectively, in the other direction of electric field.
Liquid crystal electrooptical apparatuses utilizing the tristable states, that is three stable states, are proposed in U.S. Pat. No. 5,046,823 filed by the present applicant.
The characteristics of an antiferroelectric liquid crystal showing the tristable states are described in more detail below.
In the ferroelectric liquid crystal element having a stabilized surface which was proposed by Clark-Lagawall, ferroelectric liquid crystal molecules show two stable states in which the molecules are uniformly oriented or aligned in one direction in the phase S*C. The molecules are stabilized in either state depending on the direction of applied electric field as shown in FIG. 2 at (a) and (b), and the state is kept even when the electric field was shut off.
Actually, however, the alignment of the ferroelectric liquid crystal molecules shows twisted two states in which directors of the liquid crystal molecules are twisted or shows a chevron structure in which layers are bent in a doglegged shape. In the chevron layer structure, switching angle becomes small, forming a cause for a low contrast, and which constitute a serious obstacle for its practical use.
On the other hand, in the liquid crystal electrooptical devices, an "anti" ferroelectric liquid crystal molecules are aligned in antiparallel, tilting in opposite direction at every adjoining layer, in the phase S*.sub.(3) showing the tristable states, and thus, the dipoles of the liquid crystal molecules are negating each other. Accordingly, the spontaneous polarization is nullified as a whole. The transmittance of the liquid crystal phase showing such molecular alignment corresponds to reference point 2 in FIG. 1(D).
Further, when a voltage sufficiently higher than a threshold value of (+) or (-) was applied, liquid crystal molecules shown in FIG. 3 at (b) or (c) are tilted in the same direction and aligned in parallel. In this state, the spontaneous polarization is produced since the dipoles are also shifted to the same direction to form a ferroelectric phase, and the transmittance of the liquid crystal phase in that state corresponds to reference points 1 and 3 in FIG. 1(D).
That is, in the phase S*.sub.(3) of the "anti" ferroelectric phase, the "anti" ferroelectric phase at the time of no-electric field and two ferroelectric phases due to the polarity of applied electric field are stabilized, and switching is carried out among tristable states of an "anti" ferroelectric phase and two ferroelectric phases, with a direct current-like threshold value. Based on the change in the alignment of liquid crystal molecules accompanied with the switching, light transmittance is changed while drawing such a double hysteresis as shown in FIG. 4.
One of the characteristics of the present invention is that a memory effect can be realized by applying a bias voltage to the double hysteresis as shown in FIG. 4 at reference point (A) and further applying a pulse voltage.
Moreover, the ferroelectric phase is stretched in terms of its layer by the application of an electric field to form a book-shelf structure. On the other hand, in the "anti" ferroelectric phase at the time of no electric field, an analogous book-shelf structure is formed. Since the layer structure switching due to the application of an electric field gives a dynamic shear to liquid crystal layers, an alignment defect is improved during driving, and thus, a good molecular alignment can be realized.
In the "anti" ferroelectric liquid crystal, since image display is performed by alternatively using both hysteresises of plus side and minus side, after-image phenomenon due to the accumulation of inner electric field based on the spontaneous polarization can be prevented.
As explained above, the "anti" ferroelectric liquid crystal can be said to be a very useful liquid crystal compound having advantages as follows:
1) Hipervelocity response is possible,
2) High contrast and wide viewing angle can be expected, and
3) Excellent alignment characteristics and memory effect can be realized.
Reports are made on the liquid crystal phase of the "anti" ferroelectric liquid crystal showing the tristable states in the following articles:
1 ) A. D. L. Chandani et al., Japanese J. Appl. Phys., 28, L-1265 (1989), and
2) H. Orihara et al., Japanese J. Appl. Phys., 29, L-333 (1990 ).
The liquid phase is called "Phase S*.sub.CA " (Antiferroelectric Smectic C phase) in association with the "anti" ferroelectric property. The phase is named "phase S*.sub.(3) " in the present specification since the liquid crystal phase performs the switching among tristable states.
The liquid crystal compounds which have the "anti" ferroelectric phase S*.sub.(3) showing the tristable states in a phase series are disclosed in Japanese Unexamined Patent Publication No. 1-316367, U.S. Pat. Nos. 5,171,471 and 4,973,738, and European Patent No. 330,491A filed by the present inventors, and in Japanese Unexamined Patent Publication No. 1-213390 filed by Ichihashi et al. Liquid crystal electrooptical devices utilizing the tristable states are proposed in Japanese Unexamined Patent Publication No. 2-40625 and U.S. Pat. No. 5,046,823.
When "anti" ferroelectric liquid crystals are applied for displays, it is difficult to satisfy all of these required performance of characteristics
1) the range of operation temperature,
2) response time,
3) spontaneous polarization, and
4) hysteresis,
with a single liquid crystal. Thus, the liquid crystals are usually used as a mixture of ten-odd kinds of liquid crystals.
Particularly, in respect of the required performance 1) above, that is the range of operation temperature, development of "anti" ferroelectric liquid crystals is desired which show a stabilized display performance characteristic at an area of lower temperatures including room temperature. However, "anti" ferroelectric liquid crystals have not yet been found which stably develop "anti" ferroelectric S*.sub.(3) phase at the area of lower temperatures including room temperature and showing a high speed response.